feat: Add mock server for DGX B200 system

This commit adds a comprehensive mock implementation for the 8-GPU
DGX B200 system with Blackwell architecture.

Features:
- 192GB memory per GPU
- Blackwell architecture with current-gen capabilities
- Complete NVML interface implementation
- Thread-safe operations with proper locking
- Realistic device properties and MIG configurations

Signed-off-by: Fabien Dupont <[email protected]>

Author: fabiendupont